One of the simplest and most common methods of driving an AC motor is applying input power directly to the motor through a switch (motor starter or contactor) in an across-the-line architecture, as shown in FIG. 2. The advantages of this architecture include simplicity, low cost, low energy losses while running, and minimal harmonics while running. Disadvantages include the motor can only run at one speed and in one direction, and switching on and off the load creates a large disturbance on the input power. The motor converts electrical energy to mechanical energy. The motor can drive a load, such as a fan or pump or some other form of machinery. In many applications it is desired to vary the output speed. Since the motor is across-the-line, it cannot be varied, so some other method must be employed. As an example, if the motor is turning a fan to create air-flow, one way to vary the air flow is to deflect the unwanted air away from the desired path. This results in wasted energy.
An AC motor can also be driven by a Variable Frequency Drive (VFD) as shown in FIG. 3. Using a VFD to drive an AC motor overcomes the disadvantages of an across-the-line architecture. The motor speed and direction can be controlled by the VFD, and there is no large disturbance on the input power when starting the motor. The variable speed control of the VFD over the motor increases the efficiency of powering the motor, versus an across-the-line method, when it is desired to vary the output energy of the motor. However, when powering the motor at the same speed and direction as if the input power were connected across-the line, the efficiency is less when using a VFD. A VFD also adds undesirable harmonics to the input power, as will be explained below.
A common VFD architecture changes AC power to DC power and then changes it back to AC power. There are variations of VFD technologies, but a common one will be described here. The incoming power is rectified to make DC power, called the DC bus, Transistors in the Inverter section are then turned on and off in a certain sequence (normally called Pulse Width Modulation, or PWM) by controlling logic which creates an AC signal on the output power leads.
There are energy losses when the VFD converts power from AC to DC and back to AC. There are voltage drops across the diodes in the converter section, which cause energy loss. There is controlling circuitry that needs power, which causes energy loss. And when the transistors in the inverter section are turned on, there are energy losses across the transistors.
Harmonics caused by a VFD are well documented. There are standards, such as IEEE 519, that have been created to limit harmonics because of the known detrimental effects. In general, a VFD, by its nature, is a non-linear load. The impedance of a non-linear load changes with the applied voltage. When a sinusoidal voltage is applied to a non-linear load, the load current will be non-sinusoidal. The non-sinusoidal currents contain harmonic currents, which then interact with the power system and can create voltage distortion. These harmonics can then affect the equipment in the power distribution and other loads.
The present invention is directed to reducing the energy losses and reducing the VFD harmonics when the motor speed as powered by the variable frequency drive closely matches the motor speed that would be achieved by connecting the motor directly to the incoming power.
It is known to use a bypass contactor to bypass the VFD and an output contactor to separate the output of the VFD from the motor. When the bypass contactor is closed and the VFD output contactor is open, this is known as bypass mode. In bypass mode, the motor is being powered through the bypass contactor in an across-the-line configuration. Bypass mode is shown in FIG. 5. When the bypass contactor is open and the VFD output contactor is closed, this is known as drive mode. In drive mode, the motor is being powered by the VFD and through the output contactor. Drive mode is shown in FIG. 6.
The present invention is directed to improvements in switching between drive mode and bypass mode.